U.S. patent number 6,433,921 [Application Number 09/783,395] was granted by the patent office on 2002-08-13 for multiwavelength pumps for raman amplifier systems.
This patent grant is currently assigned to Onetta, Inc.. Invention is credited to G. Victor Treyz, Yongan Wu.
United States Patent |
6,433,921 |
Wu , et al. |
August 13, 2002 |
Multiwavelength pumps for raman amplifier systems
Abstract
Multiwavelength Raman pumps for Raman amplifiers are provided.
The multiwavelength Raman pumps may be based on semiconductor
devices that have multiple source regions, each of which handles
pump light at a different wavelength. An optical coupler such as a
lens and isolator arrangement or an integral fiber lens may be used
to couple pump light from the multiwavelength Raman pump into a
fiber. A depolarizer may be used to depolarize the Raman pump light
provided by the Raman pump. Gratings may be used to define the
lasing wavelengths for the Raman pump. A number of tunable sources
may be used on the semiconductor device. A fiber Bragg grating may
be used to form an external cavity or coupled cavity arrangement
for the semiconductor device.
Inventors: |
Wu; Yongan (San Jose, CA),
Treyz; G. Victor (San Carlos, CA) |
Assignee: |
Onetta, Inc. (Sunnyvale,
CA)
|
Family
ID: |
26948243 |
Appl.
No.: |
09/783,395 |
Filed: |
February 15, 2001 |
Current U.S.
Class: |
359/334;
359/341.31; 359/341.33 |
Current CPC
Class: |
H01S
3/1022 (20130101); H01S 3/094073 (20130101); H01S
3/094096 (20130101); H01S 3/09415 (20130101); H01S
3/302 (20130101); H01S 5/141 (20130101) |
Current International
Class: |
H01S
3/102 (20060101); H01S 3/094 (20060101); H01S
3/30 (20060101); H01S 3/0941 (20060101); H01S
5/14 (20060101); H01S 5/00 (20060101); H01S
003/00 () |
Field of
Search: |
;359/334,341.31,341.33 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 99/66607 |
|
Dec 1999 |
|
WO |
|
WO 00/49721 |
|
Aug 2000 |
|
WO |
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WO 00/73849 |
|
Dec 2000 |
|
WO |
|
Other References
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km NDSF Employing Distributed Raman Amplification and Active Gain
Flattening" Electronics Letters, vol. 37, No. 1, pp. 43-45 (Jan. 4,
2001). .
Emori et al. "Cost-Effictive Depolarization Diode Pump Unit
Designed for C-band Flat Gain Raman Amplifiers to Control EDFA Gain
Profile" pp. 106-108. .
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1.43.mu.m-or 1.48.mu.m-Pumped Raman Amplification" OSA Optical
Amplifiers and their Applications, vol. 30, pp. 101-105 (1999).
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WDM Applications" 25th Optical Fiber Communication Conference,
Technical Digest, pp. 178-180 (Mar. 7, 2000). .
Moller et al. "Mode Stabilized Technique for the Multifrequency
Laser" 25th Optical Fiber Commnuication Conference, Technical
Digest, pp. 187-189 (Mar. 7, 2000). .
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with Integrated MMI Combiner, SOA, and EA-Modulator" 25th Optical
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7, 2000). .
Doerr et al. "Chromatic Focal Plane Displacement in the Parabolic
Chirped Waveguide Grating Router" IEEE Photonics Technology
Letters, vol. 9, No. 5, (May 5, 1997). .
Masuda "Review of Wideband Hybrid Amplifiers" 25.sup.th Optical
Fiber Communication Conference, Technical Digest, pp. 2-4 (Mar, 7,
2000). .
Lewis et al. "Low-Noise High Gain Dispersion Compensating Broadband
Raman Amplifier" 25.sup.th Optical Fiber Communication Conference,
Technical Digest, pp. 5-7, (Mar. 7, 2000). .
Fludger et al. "Inline Loopbacks for Improved OSNR and Reduced
Double Rayleigh Scattering in Distributed Raman Amplifiers" OFC.
.
Stentz "Progress on Raman Amplifiers" OFC '97 Technical Digest, p.
343. .
Hansen et al. "Raman Amplification for Loss Compensation in
Dispersion Compensating Fibre Modules" Electronics Letters, vol.
34, No. 11, pp. 1136-1137, May 28, 1998. .
Emori et al. "Broadband Lossless DCF using Raman Amplification
Pumped by Multichannel WDM Laser Diodes" Electronics Letters, vol.
34, No. 22, Oct. 29, 1998. .
Neilson et al. "10 Gbit/s Repeaterless Transmission at 1.3 .mu.m
with 55.1-dB Power Budget using Raman Post and Preamplifier" OFC
'98 Technical Digest, pp. 52-53. .
Stentz et al. "Raman Amplifier with Improved System Performance"
OFC '96 Technical Digest, pp. 16-17. .
Kitamura et al. "Angled Facet S-Bend Semiconductor Optical
Amplifiers for High-Gain and Large-Extinction Ratio" IEEE Photonics
Technology Letters, vol. 11, No. 7 (Jul., 1999). .
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Integrated on a Passive Active Resonant Coupler (PARC) Platform"
IEEE Photonics Technology Letters, vol. 12, No. 7, Jul.
2000..
|
Primary Examiner: Hellner; Mark
Attorney, Agent or Firm: Fish & Neave Treyz; G.
Victor
Parent Case Text
This application claims the benefit of provisional patent
application No. 60/260,884, filed Jan. 12, 2001.
Claims
What is claimed is:
1. A multiwavelength Raman pump that pumps optical fiber in a
fiber-optic communications network to produce Raman gain for
optical signals, comprising: a semiconductor device that produces
Raman pump light at multiple different pump wavelengths for pumping
the fiber; and an optical coupler for coupling the pump light into
a fiber, wherein the semiconductor device has a back facet through
which a portion of the pump light at each of the multiple pump
wavelengths exits, the pump further comprising a monitor for
monitoring the portion of the pump light that exits the back
facet.
2. A multiwavelength Raman pump that pumps optical fiber in a
fiber-optic communications network to produce Raman gain for
optical signals, comprising: a semiconductor device that produces
Raman pump light at multiple different pump wavelengths for pumping
the fiber; and an optical coupler for coupling the pump light into
a fiber, wherein the semiconductor device has a back facet through
which a portion of the pump light at each of the multiple pump
wavelengths exits at a oblique angle, the pump further comprising a
monitor for monitoring the portion of the pump light that exits the
back facet.
3. A multiwavelength Raman pump that pumps optical fiber in a
fiber-optic communications network to produce Raman gain for
optical signals, comprising: a semiconductor device that produces
Raman pump light at multiple different pump wavelengths for pumping
the fiber; an optical coupler for coupling the pump light into a
fiber; and a depolarizer that depolarizes the Raman pump light from
the semiconductor device.
4. A multiwavelength Raman pump that pumps optical fiber in a
fiber-optic communications network to produce Raman gain for
optical signals, comprising: a semiconductor device that produces
Raman pump light at multiple different pump wavelengths for pumping
the fiber; an optical coupler for coupling the pump light into a
fiber; and an external power stage that optically amplifies the
Raman pump light from the semiconductor device.
Description
BACKGROUND OF THE INVENTION
The present invention relates to fiber-optic communications
networks, and more particularly, to multiwavelength pump systems
for Raman amplifiers in fiber-optic communications networks.
Fiber-optic networks are used to support voice and data
communications. In optical networks that use wavelength division
multiplexing, multiple wavelengths of light are used to support
multiple communications channels on a single fiber.
Optical amplifiers are used in fiber-optic networks to amplify
optical signals. For example, optical amplifiers may be used to
amplify optical data signals that have been subject to attenuation
over fiber-optic links. A typical amplifier may include
erbium-doped fiber coils that are pumped with diode lasers. Raman
amplifiers have also been investigated. Discrete Raman amplifiers
may use coils of fiber to provide Raman gain. Distributed Raman
amplifiers provide gain in the transmission fiber spans that are
used to carry optical data signals between network nodes.
The fiber in Raman amplifiers may be pumped by single-wavelength
sources. However, the Raman gain spectrum produced by a
single-wavelength source often does not have the spectral shape
that is desired.
Amplifier systems with non-flat gain spectra amplify optical
signals on channels at different wavelengths by different amounts.
This is often not acceptable, particularly in communications links
with a number of cascaded amplifiers. Moreover, other non-flat
spectral shapes may be desired.
The gain spectrum of a Raman amplifier may be modified using a
spectral filter. For example, a gain equalization filter may be
used to produce a relatively flat gain spectrum by introducing
optical losses that compensate for the non-flat shape of the Raman
gain spectrum. However, the optical losses associated with using
the filter consume optical power and tend to increase the noise
figure of the Raman amplifier.
Another approach for pumping Raman amplifiers involves using a
Raman pump source based on multiple diode laser pumps, each of
which operates at a different pump wavelength. With this type of
approach, the diode laser pumps are each driven at an appropriate
current to provide a Raman gain contribution. The overall gain of
the Raman amplifier is determined by the Raman gain contributions
of each of the individual Raman pump lasers.
If a sufficient number of diode laser pumps are used, the overall
gain of the Raman amplifier may be made flat. Because gain
equalization filters are avoided, the noise figure of the Raman
amplifier may be improved. However, coupling the pump light from
each of the individual diode lasers into a single fiber for use in
a Raman amplifier may be complex, bulky, and costly.
It is therefore an object of the present invention to provide Raman
pumps that produce multiple Raman pump wavelengths.
SUMMARY OF THE INVENTION
This and other objects of the invention are accomplished in
accordance with the present invention by providing multiwavelength
light sources that may be used as Raman pumps for Raman amplifiers.
The Raman amplifiers based on the multiwavelength pumps may be used
in fiber-optic communications networks having communications links
that support channels operating at one or more different
wavelengths. The Raman amplifiers may be based on distributed or
discrete Raman amplifier arrangements. Raman gain may be provided
by pumping fiber with the multiwavelength Raman pump. The fiber may
include one or more coils of fiber such as dispersion-compensating
fiber, may be a span of transmission fiber, or may be any suitable
combination of coils and transmission fiber spans.
The gain spectrum produced by pumping the fiber in a Raman
amplifier with the multiwavelength Raman pump may be flat or may
have another desired spectral shape. The Raman amplifier may have a
control unit. The control unit may be used to control the operation
of the Raman pump. For example, the control unit may be used to
adjust the pump power produced at each of the pump wavelengths to
produce the desired spectral shape for the Raman gain. The control
unit may be used to adjust the pump power produced at each of the
pump wavelengths to produce the desired spectral Raman gain shape
for different types of gain fibers.
Optical monitoring equipment may be used to measure optical signals
on the fiber-optic communications link. The optical signal
measurements may be used by the control unit in adjusting the pump
powers produced by the multiwavelength Raman pump. Monitoring
equipment may also be used to measure the pump powers at each of
the pump wavelengths. The optical monitoring equipment may be
integrated with the pump module.
A power amplifier stage may be used to increase the optical power
from the Raman pump that is used to pump the fiber in the Raman
amplifier. The power amplifier stage may be an external
semiconductor optical amplifier or a fiber amplifier or may be a
semiconductor optical amplifier stage that is integrated with the
Raman pump.
The multiwavelength Raman pump may be based on a semiconductor
device. The device may have multiple waveguide gain sections with
different distributed feedback gratings for providing optical
feedback at different pump wavelengths. Light from each of the
waveguide gain sections may be combined using a multiplexer such as
a wavelength multiplexer or a simple Y-junction coupler on the
semiconductor device.
An optical coupler such as a lens and isolator arrangement or an
integral fiber lens may be used to couple pump light from the
multiwavelength Raman pump into a single fiber. The single fiber
may be coupled to a fiber that is to be Raman-pumped to produce
Raman gain using a pump coupler.
The semiconductor device may be mounted on a heat sink. A
temperature sensor may be used to monitor the heat sink
temperature. A thermoelectric cooling element may be used to
maintain the heat sink and semiconductor device at a desired
temperature.
A depolarizer may be used to depolarize the Raman pump light
provided by the Raman pump. This allows unpolarized Raman gain to
be produced in the Raman-pumped fiber.
Gratings may be used to define the lasing wavelengths for the Raman
pump. The gratings may be chirped. A relatively small number of
grating periods may be used in a grating to broaden the pump
linewidth.
A number of tunable sources may be used on a single semiconductor
device to provide the multiple pump wavelengths. Tunable sources
may be based on waveguide gain sections that have multiple grating
regions, each of which has a grating with a different periodicity.
Tunable sources may also be provided that use multiple waveguide
layers. The wavelength of light produced by such a layered
structure may be selected by adjusting the drive current through a
grating region on the structure.
The active region on the semiconductor Raman pump device may be
provided using multiple quantum wells. The gain spectra of the
multiple quantum wells may be configured to provide gain peaks in
the vicinity of each of the pump wavelengths.
The active region on the semiconductor Raman pump device may be
provided using multiple quantum wires or quantum dots. The gain
spectra of the multiple quantum wires or quantum dots may be
configured to provide gain peaks in the vicinity of each of the
pump wavelengths.
A fiber Bragg grating may be used to form an external cavity or
coupled cavity arrangement for the semiconductor device. The fiber
may have multiple gratings, each of which corresponds to a
different pump wavelength.
Further features of the invention and its nature and various
advantages will be more apparent from the accompanying drawings and
the following detailed description of the preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an illustrative fiber-optic
communications link including Raman amplifier equipment in
accordance with the present invention.
FIG. 2 is a schematic diagram of illustrative amplifier equipment
including a distributed Raman amplifier and an erbium-doped fiber
amplifier in accordance with the present invention.
FIG. 3 is a schematic diagram of an illustrative Raman amplifier
based on discrete coils of Raman-pumped fiber in accordance with
the present invention.
FIG. 4 is a schematic diagram of illustrative amplifier equipment
having an optical channel monitor in accordance with the present
invention.
FIG. 5 is a graph of an illustrative Raman gain spectrum for a
Raman amplifier pumped at three wavelengths to produce a flat gain
spectrum in accordance with the present invention.
FIG. 6 is a graph of an illustrative Raman gain spectrum for a
Raman amplifier pumped at three wavelengths to produce a tilted
gain spectrum in accordance with the present invention.
FIG. 7 is a diagram of an illustrative multiwavelength Raman pump
based on a semiconductor device that produces pump light at
multiple wavelengths in accordance with the present invention.
FIG. 8 is a diagram of an illustrative Y-branch coupler that may be
used to combine pump light at different wavelengths on the
semiconductor device in accordance with the present invention.
FIG. 9 is a diagram of an illustrative multiwavelength Raman pump
semiconductor device that includes a semiconductor optical
amplifier section for power amplification in accordance with the
present invention.
FIG. 10 is a diagram showing how a multiwavelength Raman pump may
be based on a multiwavelength semiconductor source coupled to an
external optical power amplifier in accordance with the present
invention.
FIG. 11 is a top view of an illustrative housing arrangement for a
Raman pump in accordance with the present invention.
FIG. 12 is a side view of the housing arrangement of FIG. 11 taken
along the line 12--12 in FIG. 11.
FIGS. 13-15 are schematic diagrams of illustrative arrangements for
depolarizing light from a multiwavelength source in accordance with
the present invention.
FIG. 16 is a top view of an illustrative semiconductor device
having multiple waveguide sources, a Y-branch coupler, and an
integral semiconductor amplifier section in accordance with the
present invention.
FIG. 17 is a side view of the illustrative semiconductor device of
FIG. 16 taken along the line 17--17 in accordance with the present
invention.
FIG. 18 is a top view of an illustrative multiwavelength
semiconductor source having a semiconductor optical amplifier
region with a flared waveguide shape in accordance with the present
invention.
FIG. 19 is a top view of an illustrative semiconductor device in
which the waveguide source regions extend to the rear facet of the
device to provide back facet monitoring capabilities in accordance
with the present invention.
FIG. 20 is a top view of an illustrative semiconductor device in
which the waveguide source regions extend to the rear facet of the
device and are curved to provide back facet monitoring capabilities
without introducing crosstalk between the source regions in
accordance with the present invention.
FIG. 21 is a diagram of an illustrative waveguide source region
that provides pump light using a full length grating region in
accordance with the present invention.
FIG. 22 is a diagram of an illustrative waveguide source region
that provides pump light using a partial grating region that is at
the rear of the waveguide source region in accordance with the
present invention.
FIG. 23 is a diagram of an illustrative waveguide source region
that provides pump light using a partial grating region that is at
the output of the waveguide source region in accordance with the
present invention.
FIG. 24 is a graph showing how a chirped grating may be used in the
multiwavelength semiconductor device in accordance with the present
invention.
FIG. 25 is a graph showing another type of chirped grating that may
be used in the multiwavelength semiconductor device in accordance
with the present invention.
FIG. 26 is a graph showing how a grating may be provided for the
multiwavelength semiconductor device using multiple grating
patterns in accordance with the present invention.
FIG. 27 is a graph showing how short gratings may be used in the
semiconductor device source regions of the multiwavelength Raman
pump in accordance with the present invention.
FIG. 28 is a diagram showing how a semiconductor device for the
multiple wavelength Raman pump may use multiple tunable sources in
accordance with the present invention.
FIG. 29 is a diagram of an illustrative tunable source based on two
grating regions that are independently controlled in accordance
with the present invention.
FIG. 30 is a graph showing how the wavelength of light that is
produced by the tunable source of FIG. 29 depends on the relative
drive currents for each of the independently-controllable source
regions of FIG. 29.
FIG. 31 is a top view of an illustrative semiconductor device
having multiple waveguide source regions each of which may use a
grating of a different periodicity to provide pump light at a
different wavelengths in accordance with the present invention.
FIG. 32 is a side view of an illustrative tunable source based on
multiple waveguide layers in accordance with the present
invention.
FIG. 33 is a graph of an illustrative gain spectrum that may be
produced by an active region that includes three different groups
of multiple quantum wells in accordance with the present
invention.
FIG. 34 is a schematic diagram of an illustrative multiwavelength
Raman pump based on a semiconductor device with multiple source
regions and an external fiber Bragg grating filter in accordance
with the present invention.
FIG. 35 is a graph showing how the arrangement of FIG. 34 may be
used to ensure that the Raman pump produces pump light at the
wavelengths determined by the fiber Bragg grating in accordance
with the present invention.
FIG. 36 is a diagram showing how a wafer of multiwavelength Raman
pump semiconductor devices may be fabricated in accordance with the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An illustrative optical communications network link 10 with Raman
amplifier equipment in an optical communications network in
accordance with the present invention is shown in FIG. 1. A
transmitter 12 may transmit information to a receiver 14 over a
series of fiber links. Each fiber link may include a span 16 of
optical transmission fiber. Fiber spans 16 may be on the order of
40-160 km in length for long-haul networks or may be any other
suitable length for use in signal transmission in an optical
communications network.
The communications link of FIG. 1 may be used to support wavelength
division multiplexing arrangements in which multiple communications
channels are provided using multiple wavelengths of light. For
example, the link of FIG. 1 may support a system with 40 channels,
each using a different optical carrier wavelength. Optical channels
may be modulated at, for example, approximately 10 Gbps (OC-192).
The carrier wavelengths that are used may be in the vicinity of
1520-1565 nm. These are merely illustrative system characteristics.
If desired, a single channel may be provided or more channels may
be provided (e.g., hundreds of channels), signals may be carried on
multiple wavelengths, signals may be modulated at slower or faster
data rates (e.g., at approximately 2.5 Gbps for OC-48 or at
approximately 40 Gbps for OC-768), and different carrier
wavelengths may be supported (e.g., wavelengths in the range of
1240-1650 nm).
Optical amplifiers 18 may be used to amplify optical signals
between successive spans of fiber 16. Optical amplifiers 18 may be
based on erbium-doped amplifier stages or other rare-earth-doped
fiber amplifier stages, may be based on semiconductor optical
amplifier stages, may be based on discrete Raman amplifier stages,
may be based on other suitable amplifier stages, or may be based on
combinations of such stages.
Fiber spans 16 may be Raman-pumped using multiwavelength Raman
pumps 20. This creates Raman gain in spans 16 that counteracts the
attenuation normally experienced along spans 16. The arrangement
shown in FIG. 1 is a counterpumping arrangement, but distributed
Raman amplifiers of this type may also be provided using copumping
or using a combination of counterpumping and copumping.
Raman pumps 20 may be polarized or unpolarized. An advantage of
using unpolarized pumps is that such pumps do not create
polarization-dependent gain in spans 16. Pump light from pumps 20
may be coupled into fiber spans 16 using pump couplers 22. Pump
couplers 22 may be, for example, wavelength-division multiplexing
(WDM) couplers or couplers based on circulators or other suitable
pump coupling arrangements.
An illustrative optical amplifier 18 with an integral
multiwavelength Raman pump 20 is shown in FIG. 2. In the example of
FIG. 2, Raman pump 20 provides light at multiple pump wavelengths
for pumping span 16. Pump 20 may be controlled by control unit 23.
Control unit 23 may also be used to control other components in
amplifier 18. The arrangement of FIG. 2 is merely illustrative.
Raman pump 20 may be provided as part of an amplifier or other
suitable optical network equipment or may be provided as
stand-alone equipment.
In the amplifier 18 of FIG. 2, optical input signals from span 16
are provided to input 24 and corresponding output signals that have
been amplified by amplifier 18 are provided at output 26. Optical
gain may be provided by rare-earth-doped fiber coils such as
erbium-doped fiber coils 28 and 30. Although only two coils are
shown in the example of FIG. 2, this is merely illustrative. One
coil, three coils, or more coils may be used if desired.
Coils 28 and 30 may be optically pumped by pumps 32 and 34. Pumps
32 and 34 may be laser diode pumps operating at 980 nm or 1480 nm
or other suitable pump wavelengths or may be any other suitable
sources of pump light. Pump couplers 36 may be used to couple pump
light from pumps 32 and 34 into coils 28 and 30. In the example of
FIG. 2, coils 28 and 30 are counterpumped. If desired, such coils
may be copumped or both copumped and counterpumped.
The illustrative amplifier 18 is somewhat simplified to avoid
over-complicating the drawing. In general, amplifier 18 may have
additional components such as components 38. In the example of FIG.
2, components 38 are located in the fiber path between pump coupler
36 and coil 30. This is merely illustrative. Components 38 may be
used at any suitable location in the fiber path in amplifier 18.
Components 38 may include taps for optical monitoring, filters such
as gain equalization filters, wavelength-division-multiplexing
couplers, circulators, isolators, attenuators,
dispersion-compensating elements, etc.
Control unit 23 may be based on any suitable control electronics
and may include one or more microprocessors, microcontrollers,
digital signal processors, programmable logic devices,
application-specific integrated circuits, digital-to-analog
converters, analog-to-digital converters, analog control circuits,
memory devices, etc.
Control unit 23 may include communications circuitry for
communicating with network equipment. For example, control unit 23
may include communications circuitry for communicating with a
network control and management system over a communications path.
The communications path may be a telemetry channel that uses a
particular wavelength on the communications link 10. The
communications path may also be based on a wireless path or may be
based on a communications arrangement in which the normal data
channels on link 10 are modulated at a low frequency and relatively
small modulation depth on top of the normal data carried on those
channels.
The network control and management system may be implemented on
suitable network computer equipment. The network control and
management system may send commands to control unit 23 that direct
amplifier 18 to establish a particular gain setting or output power
setting. The network control and management system may adjust the
settings of pump 20 and may adjust other amplifier settings.
Control unit 23 may assist in the gathering of data on the
operation of amplifier 18. For example, control unit 23 may be used
to gather information on the pump powers produced at the different
pump wavelengths in pump 20. The status and operational data
collected by control unit 23 may be provided to the network control
and management system by the control unit over the communications
link.
Control unit 23 may be used to suppress gain transients due to
fluctuations in the input powers of the signals provided to input
24. Any suitable gain transient suppression arrangement may be
used. As an example, control unit 23 may use optical taps and
monitoring circuitry to measure input and output powers in
amplifier 18. The measured input and output powers may be used to
determine the amplifier gain. Control unit 23 may adjust the powers
of the pumps in amplifier 18 to ensure that the gain is maintained
at a constant level.
As another example, an input tap may be used to measure input
signal powers. The pump powers generated by the pumps in amplifier
18 may be adjusted based on the measured input power. If desired,
gain transient control techniques may be used that involve input
and output spectral filters. Such filters may modify the entire
spectrum of the tapped input and output signals or may be used to
make power measurements for a particular channel or channels. The
modified measured powers or the power of the particular channel or
channels may be used in a feedback control scheme or other suitable
control scheme for adjusting the pump powers.
An illustrative amplifier 18 that is based on discrete Raman-pumped
fiber coils is shown in FIG. 3. In amplifier 18 of FIG. 3, optical
signals may be amplified by Raman fiber coils 40 and 42. Any
suitable optical fiber may be used as Raman fiber 40 and 42. For
example, fiber 40 and 42 may be small-core fiber or
dispersion-compensating fiber or other suitable fiber for producing
Raman gain in a coil inside an amplifier or other network
equipment. An advantage of using dispersion-compensating fiber for
fiber coils 40 and 42 is that such fiber may compensate for
chromatic dispersion on communications link 10.
Multiwavelength Raman pumps 20 of FIG. 3 are shown as being used in
pumping configurations in which coils 40 and 42 are both copumped
and counterpumped. This is merely illustrative. Raman pumps 20 may
also be used in counterpumped configurations or in copumped
configurations. Any suitable number of Raman-pumped coils may be
used. If desired, other gain stages (e.g., erbium-doped fiber gain
stages, semiconductor optical amplifier gain stages, etc.) may be
used in combination with Raman-pumped coils such as coils 40 and
42.
Control unit 23 may control Raman pumps 20 to adjust the spectral
shape of the Raman gain produced by pumps 20 and to suppress gain
transients, as described in connection with FIG. 2. Control unit 23
may also be used to control Raman pumps 20 to adjust the spectral
shape of the Raman gain produced by pumps 20 for different types of
fiber in coils 40 and 42.
As shown in FIG. 4, an optical channel monitor may be used to
measure optical signals in an amplifier 18 or elsewhere along a
link 10. Amplifier equipment 44 of FIG. 4 includes a
multiwavelength Raman pump 20 that pumps transmission fiber 16
through pump coupler 22. Optical gain for the signals on span 16 is
also provided by gain stage 46. Gain stage 46 may be an
erbium-doped fiber amplifier or other suitable optical amplifier.
Gain stage 46 may include discrete coils of Raman-pumped fiber that
are pumped with multiwavelength Raman pumps 20. Gain stage 46 and
the other equipment in FIG. 4 may be provided as a stand-alone
amplifier or may be part of an amplifier or other suitable
equipment.
Taps such as taps 48 and 50 may be used to tap optical signals
traveling in the fiber path of equipment 44. Tapped signals may be
provided to optical channel monitor 54 over fibers 56 and 58.
Optical channel monitor 54 may contain spectrum analyzing equipment
that measures the power of the optical signals. In the illustrative
example of FIG. 4, tap 48 may be used to measure the input signal
powers for stage 46 and tap 50 may be used to measure output signal
powers. With this arrangement, optical channel monitor 54 may
measure the gain spectrum of gain stage 46 and may measure the
output power of the distributed Raman amplifier based on span 16
and Raman pump 20.
Optical channel monitor 54 may provide information on the spectrum
measurements and other optical signal measurements that have been
made to control unit 23. Control unit 23 may be provided as part of
equipment 44 as shown in FIG. 4 or may be provided as part of an
amplifier 18 or other suitable equipment. Control unit 23 may
control the pump powers produced at each of the multiple pump
wavelengths provided by Raman pump 20 or Raman pumps 20 in stage 46
based on the information provided by optical channel monitor 54.
For example, control unit 23 may control the pump powers of such
Raman pumps to adjust the Raman gain spectrum in span 16 or stage
46.
An illustrative Raman gain spectrum 60 that may be produced when a
multiwavelength Raman pump 20 is used to pump a transmission fiber
16 or a coil of fiber is shown in FIG. 5. In the example of FIG. 5,
three different pump wavelengths .lambda..sub.P1, .lambda..sub.P2,
and .lambda..sub.P3 have been used to produce spectrum 60. Each
pump wavelength makes a contribution 61 to the gain spectrum 60. As
shown in FIG. 5, spectrum 60 may be made relatively flat, even
without using gain flattening filters. Further gain flattening may
be achieved by using more pump wavelengths (e.g., using 4, 5, 6, .
. . 10, 11, or more different pump wavelengths). The different pump
wavelengths are typically separated in wavelength by at least 1 nm
or 2 nm or more typically by at least 5 nm, 10 nm, 20 nm, or
more.
As shown in FIG. 6, the pump powers of the different pump
wavelengths may be adjusted to provide a tilted Raman gain spectrum
60, rather than the flat spectrum 60 of FIG. 5. The tilted spectrum
60 of FIG. 6 is merely illustrative. Any suitable spectral shape
may be obtained by adjusting the pump powers of pump 20 if desired.
Pump power tends to be transferred from shorter wavelength pumps to
longer wavelength pumps due to Raman gain. If desired, these
pumping effects may be taken into account when adjusting the pump
powers.
An illustrative multiwavelength Raman pump 20 is shown in FIG. 7. A
semiconductor device 62 may be used to generate pump light at
multiple wavelengths (.lambda..sub.1, .lambda..sub.2,
.lambda..sub.3, . . . .lambda..sub.N,). Semiconductor device 62 may
be based on any suitable light-generating semiconductor material
system. For example, semiconductor device 62 may be based on
structures formed from InP, InGaAsP, InGaAlAs or other suitable
semiconductors. Semiconductor device 62 may have active regions
based on bulk materials, multiple quantum well (MQW) structures,
quantum wires, quantum dots, or any other suitable semiconductor
structures for generating pump light or combinations of such
structures. Optical confinement may be provided in the vertical
dimension by controlling the index of refraction of the
semiconductor layers used in device 62. Lateral optical confinement
may be provided using waveguide structures such as ridge waveguides
or buried heterostructure waveguides.
The diagram of FIG. 7 is a top view of device 62. Waveguides are
represented by lines 64 and line 66. The regions represented by
lines 64 may be source regions that use gratings or other
structures to produce light at particular wavelengths. The light
from each of these source regions may be combined into a single
path (represented by line 66) using an optical multiplexer or other
suitable optical combiner 68.
Optical multiplexer 68 may be a wavelength division multiplexing
(WDM) structure such as an arrayed waveguide (AWG) structure, a
splitter (e.g., a multibranch Y structure), a multimode
interference (MMI) structure, or any other suitable optical
multiplexing structure.
Multiwavelength pump light that exits device 62 at the end of path
66 may be coupled into fiber 70 using optical coupler 72. Optical
fiber 70 may be used to provide the pump light to an appropriate
pump coupler 22 for pumping a fiber span 16 or coil of Raman-pumped
fiber. Optical coupler 72 may be a lens, a pair of lenses with an
intermediate isolator, or any other suitable lens or optical
coupling system for coupling pump light exiting a waveguide at the
edge of device 62 into optical fiber 70. If desired, an integral
fiber lens at the tip of fiber 70 may be used as the optical
coupler 72 or may be used in combination with a separate optical
coupler 72 such as a lens system.
The waveguides of the source regions represented by lines 64 may
extend to the back facet 74 (rear face) of semiconductor device 62.
Light may exit each of the waveguides at the back facet, as
indicated by dotted lines 76. A monitor 78 may be used to detect
this light. Monitor 78 may be any suitable device for measuring the
powers of the signals exiting device 62. For example, monitor 78
may be a photodiode array or other device having individual
detectors 80, each of which measures the power exiting a respective
one of the source region waveguides 64. If desired, monitor 78 may
be based on a charge-coupled device or any other suitable detector
arrangement.
An illustrative semiconductor device arrangement having a
multiplexer 68 based on a Y-branch waveguide structure 82 is shown
in FIG. 8. Light is produced by the active semiconductor layers in
source regions 64. Two source regions are shown in FIG. 8 and some
of the other drawings, but any suitable number of source regions
may be used. Each source region 64 may produce or handle pump light
at a different pump wavelength. Structure 82 combines light from
the source regions 64 and provides the combined light to waveguide
structure 66. Light exits waveguide structure 66 at exit 84.
As shown in FIG. 9, semiconductor device 62 may include an optical
amplifier section 86 that amplifies the pump light signals at all
pump wavelengths. Amplifier section 86 may be a semiconductor
optical amplifier gain stage that is based on the same
semiconductor active regions that are used to produce pump light in
source regions 64. Amplifier section 86 may be used to
simultaneously increase the power of all of the pump wavelengths
from source regions 64. The drive currents used to control each of
source regions 64 may be independently adjusted by control unit 23.
This allows the relative pump power for each wavelength in the
multiwavelength pump 20 to be adjusted independently. The drive
current for amplifier section 86 may be also be independently
adjusted if desired.
As shown in FIG. 10, an external optical amplifier stage 86 may be
used to amplify the optical signals at the different pump
wavelengths that are produced by source regions 64. Amplifier stage
86 may be a semiconductor optical amplifier stage, a Raman fiber
amplifier stage, a rare-earth-doped fiber amplifier stage, or any
other suitable amplifier stage.
Any suitable housing arrangement may be used for Raman pump 20. One
suitable arrangement is shown in FIGS. 11 and 12. As shown in the
top view of FIG. 11, the various components of pump 20 may be
housed in a package case 88. Case 88 may be formed of metal or
other suitable materials. Within case 88, semiconductor device 62
may be mounted on a heatsink 90. Heatsink 90 may be formed from
copper, aluminum, alumina, silicon carbide, aluminum nitride, or
any other suitable heatsink materials. Heatsink 90 removes excess
heat from device 62 during operation, and allows the temperature of
device 62 to be controlled.
Monitor 78, optical coupler 72, and fiber 70 may also be mounted in
case 88. As shown in the side view of FIG. 12, monitor 78, device
62, optical coupler 72, and fiber 70 may be mounted in vertical
alignment in case 88. Most of the pump light from device 62 exits
device 62 in the forward direction and is coupled into fiber 70 by
pump coupler 72. A fraction of the pump light may also exit device
62 in the backwards direction for monitoring by monitor 78.
If desired, a thermoelectric cooling (TEC) element 92 may be used
to heat or cool heatsink 90. If desired, a thermistor 94 or other
suitable temperature sensor may be mounted on heatsink 90 or at
another suitable location within the pump housing. Temperature
information from thermistor 94 may be provided to control unit 23
or other suitable control electronics. This information may be used
in controlling the temperature of heatsink 90 and device 62. For
example, a feedback scheme may be used in which the temperature
information from thermistor 94 is used to maintain the temperature
of heatsink 90 at a desired level. If desired, the thermoelectric
cooler may be used to adjust the temperature of device 62 to adjust
the lasing wavelengths of device 62.
The light exiting semiconductor device 62 may be polarized (e.g.,
linearly polarized). If polarized light is used to Raman pump fiber
16 or coils 40 and 42, the Raman gain produced in the fiber may be
polarization sensitive. This is generally not desired. Accordingly,
a depolarization scheme may be used to depolarize the light exiting
device 62 before the light is provided to fiber 70. For example,
the pump light from two Raman pumps 20 may be combined with a
polarization beam combiner. The combined pump light may be used to
provide Raman gain with a reduced polarization dependence.
As shown in FIG. 13, a depolarizer 94 may be used to depolarize the
light exiting device 62. Depolarizer 94 may be based on
polarization maintaining fiber that is oriented at 45.degree. with
respect to the angle of the linearly-polarized light exiting device
62. With this type of arrangement, linearly-polarized light from
device 62 may be launched equally along the slow and fast axes of
the polarization maintaining fiber. The polarization-maintaining
fiber may be long enough with respect to the coherence length of
semiconductor device 62 to ensure that when the light from the slow
and fast axes exits the end of the polarization-maintaining fiber
and is recombined, the polarization dependence of the light is
removed. This is merely one illustrative example of a suitable
depolarization device that may be used to remove polarization from
the light exiting device 62. Any other suitable depolarizing
arrangement may be used if desired.
In the illustrative arrangement shown in FIG. 13, depolarizer 94 is
located between device 62 and an external power amplifier stage 86.
If desired, depolarizer 94 may be located after stage 86, as shown
in FIG. 14.
Another suitable arrangement is shown in FIG. 15. In the FIG. 15
example, power amplifier stage 86 is provided on the same
semiconductor device 62 that is used to produce the pump
wavelengths for pump 20. Depolarizer 94 is used to produce
unpolarized light from the polarized light that exits device 62
after stage 86.
The front and back facets of device 62 may be provided with
coatings. As shown in FIG. 16, for example, the front facet 96 of
device 62 may be provided with an antireflection (AR) coating 98 or
other suitable low-reflectivity coating. Coating 98 may be formed
from one or more dielectric layers or any other suitable coating
materials. Using an AR coating 98 on front facet 96 may improve the
efficiency of device 62 in providing pump light. If desired, no
coating need be used on front facet 98.
Back facet 74 may be left uncoated or may be provided with a
coating 100. Coating 100 may be formed from one or more dielectric
layers or metal layers or any other suitable coating materials.
Coating 100 may be, for example, a high-reflectivity coating that
provides a broadband reflectivity of more than 80% or more than 95%
or any other suitable amount of reflectivity. The high-reflectivity
coating 100 may reflect light from source regions 64 back into
source regions 64. Using the high-reflectivity coating on back
facet 74 may therefore improve the efficiency of device 62 in
generating pump light. The high-reflectivity coating on back facet
74 may also help to broaden the spectral linewidth of the pump
light. This increased linewidth may tend to reduce undesirable
non-linear effects such as four-wave-mixing in the Raman
amplifier.
Contact pads 102 may be provided that allow drive currents to be
applied to source regions 64 and amplifier section 86. The areas
underlying contact pads 102 are active semiconductor regions that
generate gain when driven with a current.
The active regions 104 are shown in the side view of FIG. 17. In
some regions of device 62 such as in passive multiplexer structures
68, it may not be desired to provide an active region 104. In these
regions, a passive optically-transparent layer 106 may be provided.
As shown in FIG. 17, layer 106 may be used to optically guide pump
light from the active regions 104 of sources 64 into the active
region 104 of amplifier stage 86. Layer 106 may be formed using any
suitable technique such as by masking regions 104, etching the
masked device 62, depositing layer 106, and removing the masks.
Waveguide cladding layers 108 may be provided above and below
active regions 104 and layer 106 to provide vertical optical
confinement. The index of refraction of layers 104 and 106 may be
relatively constant or may be graded. Layers 108 may have lower
indices of refraction than layers 104 and 106. Layers 108, 104, and
106, may be based on InP, InGaAsP, InGaAlAs or other suitable
semiconductor material systems. Layer 106 may be based on a
discrete waveguide or other components that are bonded to
semiconductor device 68 to guide the pump light.
Gratings such as gratings 110 may be provided for each source
region 104. The gratings may be provided, for example, in cladding
layers such as layers 108. The gratings typically are formed at the
interface between two layers with different indices of refraction.
Gratings may also be formed by creating periodic structures in the
semiconductor that modify or create gain in the semiconductor in a
periodic pattern.
The gratings 110 may be different for each source region 64. The
grating 110 that is associated with a particular source region
helps define the wavelength of light that is produced by that
source region during operation of device 62.
Each period in a grating 110 forms a source of feedback for light
in the associated source region 64. Although the reflection from
any one period of grating 110 is relatively small, the combined
effect of each of the multiple periods in grating 110 produces a
substantial reflection from grating 110 at the desired wavelength
for that source region 64. Gratings 110 may be distributed Bragg
grating reflectors, full or partial distributed feedback gratings,
or any other suitable grating structures.
As shown in FIG. 18, semiconductor device 62 may have an
amplification stage 86 that is based on an adiabatically-tapered
waveguide structure or a flared gain structure 112 (e.g., a
structure using a flared contact pad without using lateral
waveguide confinement). Structure 112 may be flared by an amount
(e.g., a 6 degree angle) that allows light from multiplexer region
68 to expand in horizontal dimension 114 as the light propagates
from multiplexer 68 to output 84. The light passing along the
flared portion 112 may expand freely in lateral dimension 114 at
the same or nearly the same rate as light would expand in a uniform
semiconductor, because flared portion 112 need not have waveguide
regions to provide lateral confinement. Cavity spoilers 115 may be
used to block light traveling in the backwards direction.
As shown in FIG. 19, the ends 116 of source region 64 may abut back
facet 74 at right angles, so that light exiting back facet 74 may
pass to monitor 78 along paths such as the paths shown by dotted
lines 118.
If desired, angled ends 120 may be used for source regions 64, as
shown in FIG. 20. With this type of approach, the ends of source
regions 64 abut back facet 74 at oblique angles, so that light
exiting back facet 74 of device 62 may pass to monitor 78 along
paths such as the paths shown by dotted lines 122 to avoid
crosstalk between different detector portions of monitor 78.
The gratings 110 that may be used to define the different pump
wavelengths produced by source regions 64 may be provided along the
entire length of the source regions 64, as shown in FIG. 21. The
back facet 74 may be uncoated. If desired, gratings 110 may be
provided at the back facet end of regions 64, as shown in FIG. 22.
The back facet 74 in the FIG. 22 arrangement may be uncoated.
Another suitable arrangement involves providing gratings 110 near
the front of the source regions 64, as shown in FIG. 23. With this
approach, back facet 74 may be uncoated or may be coated with a
high-reflectivity coating 100.
Gratings 110 may have equal periods. Gratings may also be chirped
(i.e., provided with different periods). A graph showing the
varying periodicity of an illustrative chirped grating is shown in
FIG. 24. In the graph of FIG. 24, the grating depth (the vertical
height of the grating perpendicular to the surface of device 62) is
plotted as a function of the distance along the grating (parallel
to the waveguide defining the associated source region 64). The
period of the grating is not constant, but varies as a function of
position. As a result, different wavelengths of light are reflected
from different portions of grating 110.
As shown in FIG. 25, a chirped grating may be provided in which
multiple periods of the grating have substantially the same period.
The period of the illustrative grating 110 of FIG. 25 is the same
for each group of three periods. If desired, a larger or smaller
number of periods may be repeated in grating 110.
Another approach for providing a chirped grating characteristic is
illustrated in FIG. 26. With this approach, one mask or
interference pattern is used to etch or otherwise impress the
grating pattern 124 on the grating 110 and another mask or
interference pattern is used to etch or otherwise impress the
grating pattern 126 on the grating 110. The patterns 124 and 126
may be impressed on the grating at the same time (e.g., when the
patterns result from interference patterns created by light) or
sequentially (e.g., when the patterns result from using masks). The
resulting grating 110 has a reflection characteristic that is a
combination of the reflection characteristics associated with
patterns 124 and 126. Although two patterns are shown as being
combined in the example of FIG. 26, any suitable number of patterns
may be combined if desired.
As shown by the graph of FIG. 27, a grating 110 may be provided
that has a relatively small number of periods (e.g., 3-30 periods).
Using a grating with a relatively small number of periods may
create a spectrally-broad reflection or feedback characteristic and
a correspondingly broad spectrum of light may be emitted from
source region 64. Such broad spectral characteristics may be
desirable for Raman pumping schemes, because sources with narrow
spectral characteristics may give rise to non-linear effects in the
pumped fiber such as stimulated Brillouin scattering or
four-wave-mixing.
A semiconductor device 62 for the multiple wavelength Raman pump 20
may use multiple tunable sources 64, as shown in FIG. 28. Each
tunable source region 64 may be tuned to produce a different
wavelength of light.
A diagram of an illustrative tunable source 64 based on two grating
regions that are independently controlled is shown in FIG. 29. The
periodicity of the grating 110 underlying each pad 102 may be
different, which allows either grating region to be activated by
applying the appropriate drive current to pads 102.
With the illustrative configuration of FIG. 29, a current I.sub.1,
may be applied to region 64 when it is desired to create gain under
pad 102a. A current I.sub.2 may be applied to region 64 when it is
desired to create gain under pad 102b.
The resulting gain spectra for source region 64 of FIG. 29 for
various different combinations of currents I.sub.1, and I.sub.2 is
shown in FIG. 30. As shown in FIG. 30, the wavelength of light that
is produced by the tunable source of FIG. 29 depends on the
relative drive currents for each of the independently-controllable
portions of source region 64 of FIG. 29.
Another suitable configuration for a multiple-wavelength pump based
on tunable source regions 64 is shown in FIG. 31. In the example of
FIG. 31, each tunable source region 64 may be based on a grating
110 with a different periodicity or with the same periodicity.
Light may be generated by source region 64a by applying current to
a portion of source region 64a through contact pad 102-3. Light may
be generated by source region 64b by applying current through
contact 102-4. The wavelengths of light that are generated by
source regions 64a and 64b are determined by the periodicity of the
corresponding grating 110. The wavelengths of light that are
generated by source regions 64a and 64b can be tuned by adjusting
the current applied through pads 102-1 and 102-2, respectively.
Although structures such as device 62 of FIG. 31 are shown with
only two source regions 64, this is merely illustrative. Any
suitable number of source regions 64 may be provided if
desired.
A side view of an illustrative tunable source region 64 based on
multiple waveguide layers 128 and 130 is shown in FIG. 32. Source
region 64 may be one of source regions 64a and 64b in FIG. 31 or
may be one of a group of source regions, each of which has a
grating 110 with the same periodicity.
As shown in FIG. 32, a current J may be applied in the vicinity of
grating 110. The magnitude of current J may be adjusted to alter
the index of refraction surrounding grating 110 so that light at
either wavelength .lambda..sub.1 or .lambda..sub.2 is coupled from
waveguide layer 128 into waveguide layer 130 by grating 110.
Residual light that is passing through layer 128 past grating 110
in direction 132 at the desired and undesired wavelengths is
absorbed by layer 128, because layer 128 provides optical
absorption in the absence of current J. The light at the desired
wavelength (.lambda..sub.1 or .lambda..sub.2) that is coupled into
waveguide layer 130 is reflected back in direction 134 by
reflective coating 100, because waveguide 130 does not absorb light
at wavelengths .lambda..sub.1 and .lambda..sub.2. In the vicinity
of grating 110, grating 110 couples this light back from layer 130
into layer 128. This light may exit device 62 through
antireflection coating 98 (or other suitable low-reflectivity
coating), as indicated by arrow 136.
The spectra produced by pump 20 may be adjusted by controlling the
thicknesses and material compositions of the multiple quantum wells
or other structures in each source region 64. A graph of an
illustrative gain spectrum that may be produced by an active region
in a region 64 that includes three different groups of multiple
quantum wells is shown in FIG. 33. A region 64 with the spectrum of
FIG. 33 may be tuned to produce light at a wavelength of
.lambda..sub.1, .lambda..sub.2, or .lambda..sub.3. A multiple
quantum well structure or other structure that produces a spectrum
of the type shown in FIG. 33 may therefore be used as the active
layer in a device 62 having source regions 64 each of which has a
grating or other structure that restricts the emitted light that is
associated with that source region 64 to .lambda..sub.1,
.lambda..sub.2, or .lambda..sub.3, respectively.
An external cavity or coupled-cavity arrangement may be used to
selectively produce pump light at different pump wavelengths, as
shown in FIG. 34. In the example of FIG. 34, light from device 62
is coupled into a fiber 138 having gratings 140 and 142 by optical
coupler 72 (e.g., a lens or an optical coupler that is integral
with the tip of fiber 138). Gratings 140 and 142 (e.g., fiber Bragg
gratings) reflect a small amount of light at appropriate
wavelengths back into waveguide 66. Multiplexer 68 directs light at
each wavelength into an appropriate source region 64. Light exiting
each source region 64 is coupled into waveguide 66 by multiplexer
68.
A graph showing how the arrangement of FIG. 34 may be used to
ensure that Raman pump 20 produces pump light at the wavelengths
.lambda..sub.1, and .lambda..sub.2 determined by the gratings 140
and 142 is shown in FIG. 35. The spectrum labeled MUX 1 corresponds
to the multiplexer pass band associated with a first leg 151 of
multiplexer 68 (or a set of such legs). The spectrum labeled MUX 2
corresponds to the pass band of a second leg 152 of multiplexer 68
(or a set of such legs). As shown in FIG. 35, gratings 140 and 142
have bandwidths of .DELTA..lambda. about the desired wavelengths
.lambda..sub.1, and .lambda..sub.2. These wavelengths lie within
the pass bands of the legs of multiplexer 68. As a result of the
wavelength selectivity of filters 140 and 142 and the wavelength
selectivity of multiplexer 68, the light produced by source regions
64a and 64b and therefore by pump 20 is centered around
.lambda..sub.1 and .lambda..sub.2 with bandwidths
.DELTA..lambda..
Semiconductor patterning techniques may be used to manufacture
wafers of devices 62 with different source regions 64. An example
is shown in FIG. 36. A pattern may be formed on wafer 144 that
opens mask holes 144 overlying source regions 64-1. After gratings
110 are formed through holes 144, holes 144 may be covered and
holes 146 may be opened that overlie source regions 64-2. Gratings
110 of a different periodicity than the gratings 110 formed through
holes 144 may then be formed. This approach may be repeated to form
any suitable number of source regions 64 having different gratings
or other structures. Any suitable masking techniques may be used to
form mask holes such as holes 144 and 146. For example, such holes
may be formed in mask layers formed from oxides, glasses,
photoresists, metals, polymers, etc. Moreover, the example of FIG.
36 is merely illustrative. Any suitable semiconductor fabrication
techniques may be used to pattern the waveguides, gratings, contact
pads, and other structures associated with devices 62 of pumps
20.
It will be understood that the foregoing is merely illustrative of
the principles of this invention, and that various modifications
can be made by those skilled in the art without departing from the
scope and spirit of the invention.
* * * * *